1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly, to a semiconductor device including a gate oxide film without any constriction and a method of manufacturing the same.
2. Description of the Related Art
A method of manufacturing an n-channel type MOS transistor (hereinafter referred to as an nMOS transistor) will now be explained as an example of a conventional semiconductor device.
A thermal oxide film is formed on a p-type silicon substrate, and a polysilicon film is formed on the thermal oxide film. The polysilicon film and the thermal oxide film are patterned using a photoresist film, thereby to form a gate electrode and a gate oxide film. After this, the photoresist film is removed therefrom. Then, ions are implanted into the surface of the silicon substrate so as to form n-type diffusion layers. Subsequently, a side wall is formed on the side wall of the gate electrode. Ions are implanted into the surface of the silicon substrate while using the gate electrode and the side wall as masks, so as to form n+-type diffusion layers for forming a source and drain areas. Heat treatment is carried out so as to activate the impurities, so that a source and drain diffusion layers each comprising the n-type diffusion layer and the n+-type diffusion layer can be formed. Then, the gate electrode and the n+-type diffusion layers are silicided, and an interlayer insulating film is formed on the silicon substrate. Furthermore, contact holes are formed in the interlayer insulating film, and wiring is formed on the n+-type diffusion layers.
According to the above method of manufacturing the semiconductor device, the gate oxide film is exposed to a removing agent when removing the photoresist film, and exposed to a cleaning agent when cleaning the silicon substrate. This causes a constriction (hollow) to be formed in the gate oxide film. If there is any constriction in the gate oxide film, an oxide film formed on the silicon substrate enters the constriction when forming a side wall. In the semiconductor device having such a defective gate oxide film, unnecessary current flows through the gate oxide film when an electric field is applied between the gate electrode and the substrate, resulting in breaking down the gate oxide film.
After the formation of the gate electrode, having formed a thick thermal oxide film, the constriction can be filled with an oxide, etc., and thus restoring the gate oxide film. In order to prevent the restored gate oxide film from being exposed to the release agent for photoresist film or the cleaning agent for silicon substrate during a process following this process of restoring the gate oxide film, it is necessary to remain the thick thermal oxide film on the silicon substrate. In this case, in the state where the thermal oxide film is formed both on a source and drain electrode areas, an ion implantation process is carried out, thus it is difficult to manufacture a semiconductor device in which a pn-junction is formed not far from the surface of the substrate.
Unexamined Japanese Patent Application KOKAI Publication No. S62-241379 discloses a method of manufacturing a semiconductor device, wherein, after the formation of a gate electrode using a photoresist film, a nitride film is formed on the side wall of a gate electrode and the photoresist film is removed therefrom. According to this method, an ion implantation process is carried out in the state where an insulating film is formed both on a source and drain electrode areas, therefore, it is difficult to form the semiconductor device wherein the pn-junction is formed not far from the surface of the substrate.
Unexamined Japanese Patent Application KOKAI Publication No. H4-58566 discloses a method of manufacturing a semiconductor device, wherein, after the formation of a gate electrode, an oxide film is formed on the side wall of the gate electrode. The thickness of the oxide film to be formed on the side wall of the gate electrode is 30 to 100 nm, i.e., it is quite thick. Thus, it is difficult to diffuse an impurity diffusion layer into the lower end of the gate electrode through heat treatment.
It is accordingly an object of the present invention to provide a semiconductor device having a gate insulating film with high reliability and a method of manufacturing the same.
Another object thereof is to provide a semiconductor device, wherein a pn-junction is formed not far from the surface of a substrate, and a method of manufacturing the same.
In order to achieve the above objects, according to the first aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
forming a first insulating film on a semiconductor substrate;
forming a conductive film on the first insulating film;
forming an etching mask with a predetermined pattern on the conductive film;
etching the conductive film using the etching mask, so as to form a gate electrode;
removing the etching mask with a removing agent;
forming a second insulating film, with which a constriction formed in the first insulating film by the removing agent is filled and which is incorporated with the first insulating film, on a circumference of the gate electrode;
forming a gate insulating film which insulates between the gate electrode and the semiconductor substrate, by removing the first insulating film using the gate electrode and the second insulating film as masks;
forming a protection film which covers at least a circumference of the gate insulating film, in order to protect the gate insulating film during a step following this step; and
implanting an impurity into the semiconductor substrate using the gate electrode, the second insulating film and the protection film as masks, and forming a source area and a drain area by performing heat treatment.
According to the above method, after the removing of the etching mask with the removing agent, having formed the second insulating film which is incorporated with the first insulating film, the constriction (hollow) made in the first insulating film by the removing agent can be filled. Accordingly, the gate insulating film without any constriction can be formed.
Having formed the protection film in the circumferential section of the restored gate oxide film, it becomes preventable that the gate oxide film is exposed to a chemical agent during a process for removing the etching mask or a process for cleaning the substrate, for example. This prevents any new constriction from being formed. Therefore, a semiconductor device having a gate oxide film with high reliability can be manufactured.
The forming the gate insulating film may include removing the first insulating film and a part of the second insulating film which corresponds to an upper surface of the gate electrode; and
the forming the protection film may include forming the protection film as to cover the first and second insulating films, and the method may comprise:
removing a part of the protection film which corresponds to the upper surface of the gate electrode, thereby to expose the upper surface of the gate electrode; and
forming a silicide layer on the upper surface of the gate electrode.
After the formation of the second insulating film, the second insulating film formed on the upper surface of the gate electrode is removed therefrom. After the formation of the protection film, the protection film is removed from the upper surface of the gate electrode. By doing this, the upper surface of the gate electrode is exposed, and a silicide layer is formed on the gate electrode, resulting in siliciding the gate electrode.
In the method of manufacturing a semiconductor device,
the semiconductor substrate may be a silicon substrate;
the first insulating film and the second insulating film may be thermal oxide films;
the conductive film may be a polysilicon film;
the etching mask may be a photoresist pattern;
the removing agent may be a removing agent for photoresist, by which agent the thermal oxide film is unwantedly etched; and
the protection film may be a nitride film which is hardly removed by the removing agent for photoresist, as compared to the thermal oxide film.
In the case of manufacturing a semiconductor device using the above elements, the gate insulating film, which includes the thermal oxide film formed on the silicon substrate, and the thermal oxide film, which is formed on the surface of the gate electrode formed of polysilicon, can easily be incorporated with each other.
The nitride film can be etched at a higher etching rate than that for the thermal oxide film. Thus, even if the nitride film is exposed to the removing agent for photoresist or the cleaning agent for silicon substrate, it is not hardly etched. This achieves sufficient protection of the gate insulating film.
The removing agent for photoresist may be a mixture of NH4OH and H2O2.
The method of manufacturing a semiconductor device, may comprise:
forming a first diffusion area by implanting an impurity into the semiconductor substrate at a first concentration and a first energy level, through utilization of the gate electrode, the second insulating film and the protection film as masks;
cleaning the semiconductor substrate using a cleaning agent, while protecting the second insulating film with the protection film;
forming a side wall film on the protection film;
implanting an impurity into the semiconductor substrate at a second concentration which is higher than the first concentration and at a second energy level higher than the first energy level, through utilization of the gate electrode, the second insulating film, the protection film and the side wall film as masks, thereby to form a second diffusion area at a high concentration which area overlaps with the first diffusion area, and which is formed in a deeper position than a position of the first diffusion area; and
diffusing and activating an impurity inside the first and second diffusion area by heat treating the semiconductor substrate, and forming a source area and a drain area each of which includes a low concentration area reaching a lower end of the gate electrode and a high concentration area connected to the lower concentration area.
It is preferred that the second insulating film be formed to a thickness of 2.0 to 7.0 nm. In this case, a constriction (follow) formed in the first insulating film can be filled without any opening.
It is preferred that the protection film be formed to a thickness of 3.0 to 8.0 nm. In this case, the restored gate insulating film can reliably be protected during any following processes.
In order to accomplish the above objects, according to the second aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:
forming a well area of an opposite conductivity type to a conductivity type of a semiconductor substrate on a surface the semiconductor substrate;
forming a first and second gate insulating films respectively on the surface of the semiconductor substrate and a surface of the well area; forming a first gate electrode on the semiconductor substrate via the first insulating film, and a second gate electrode on the well area via the second gate insulating film;
forming a first and second protection films which respectively covers at least circumferences of the first and second gate insulating films;
forming a first diffusion area, by covering the well area with a first mask and implanting a first impurity into the semiconductor substrate through utilization of the first mask, the first gate electrode and the first protection film as masks;
removing the first mask with a removing agent while protecting the first and second gate insulating films respectively with the first and second protection films;
forming a second diffusion area, by covering the semiconductor substrate with a second mask and implanting a second impurity into the well area through utilization of the second mask, the second gate electrode and the second protection film as masks;
removing the second mask with a removing agent, while protecting the first and second gate insulating films respectively with the first and second protection films; and
diffusing and activating an impurity inside the first and second diffusion areas by heat treating the semiconductor substrate, forming a first and second drain areas each including the first diffusion area reaching a lower end of the first gate electrode, and forming a second source and drain areas each including the second diffusion area reaching a lower end of the second gate electrode.
According to the above method, during the formation of a CMOS device, the gate insulating film formed in the NMOS transistor area or the pMOS transistor area can be protected by the protection film. According to this method, the gate insulating film can not be exposed to the removing agent during the process for removing the mask. Thus, the gate insulating film with high reliability can be formed.
In order to accomplish the above objects, according to the third aspect of the present invention, there is provided a semiconductor device comprising:
a semiconductor substrate;
a gate insulating film formed on the semiconductor substrate;
a gate electrode formed on the gate insulating film;
an insulating film which is formed around the gate electrode and incorporated with the gate insulating film;
a protection film which is formed at least a circumference of the gate insulating film;
a source area and a drain area which are formed on the semiconductor substrate and reach a lower end of the gate electrode.
The semiconductor device of the present invention may comprise:
a side wall which is formed on a periphery of the gate electrode as to cover the insulating film and the protection film; and
the source and drain areas which are formed on a surface of the semiconductor substrate and both of which respectively include a low concentration area reaching the lower end of the gate electrode and a high concentration area connected to the lower concentration area.
In order to accomplish the above objects, according to the fourth aspect of the present invention, there is provided a semiconductor device comprising: a semiconductor substrate;
a well area which is formed on a surface of the semiconductor substrate and which is of an opposite conductivity type to a conductivity type of the semiconductor substrate;
a first and second gate insulating films which are respectively formed on the surface of the semiconductor substrate and on a surface of the well area;
a first gate electrode which is formed on the semiconductor substrate via the first gate insulating film;
a second gate electrode which is formed on the well area via the second gate insulating film;
a first and second protection films which respectively cover at least circumference of the first and second gate insulating films;
a first source and drain areas each of which includes a first diffusion area and each of which is formed on the surface of the semiconductor substrate; and
a second source and drain areas each of which includes a second diffusion area and each of which is formed on the surface of the well area.